The production of wool glass fibers by means of the rotary process is well known. In general, molten glass is fed into a spinner which revolves at high speeds. The spinner has a peripheral wall containing a multiplicity of orifices. Molten glass passed by centrifugal force through the orifices of the peripheral wall forms small diameter molten glass streams. Positioned circumferentially about the spinner is an annular blower for turning the fibers downwardly and, in some cases, for further or secondary attenuation of the original or primary fibers to produce fibers of smaller diameter. As the streams of molten glass are emitted from the orifices, they are still sufficiently nonviscous that surface tension forces pull or shape each of the molten streams into substantially circular cross-sections, regardless of the cross-sectional shape of the streams as they are emitted from the orifices. Further, rotary fiberizers are typically equipped with annular burners or other sources of hot gases for secondary attenuation of the primary fibers; these hot gases keep the glass sufficiently fluid or nonviscous that fibers of substantially circular cross-section result.
The production of textile or continuous glass fibers by mechanically drawing molten streams of glass from orifices in the bottom wall of a bushing or feeder is also well known. Non-uniformities in the roundness of the molten streams tend to be corrected by surface tension forces prior to the cooling and hardening of the molten streams into glass fibers. Thus, as in the case of wool glass fiber production, it has not been possible to produce significantly non-circular continuous fibers using shaped orifices in a bushing.
There has long been a need for producing fibers, both in the rotary process and in the continuous fiber process, that have significantly non-circular cross-sections. With respect to reinforcement of resin matrices, such non-circular fibers would be useful in imparting greatly increased transverse strength and improved shear strength qualities. Non-circular fibers for use as insulation materials would be advantageous in that the increased surface area per unit volume of glass would lower the thermal conductivity of insulation made from such fibers.
A measure of the non-circularity of mineral fibers is the "mod ratio", which is defined as the ratio of the diameter of the smallest circle into which the fiber cross-section fits to the diameter of the largest circle which can fit inside the fiber cross-section. As employed herein, fibers having a mod ratio of less than 1.2 are referred to as circular fibers; fibers having a mod ratio greater than or equal to 1.2 are referred to as non-circular fibers.
There has also been a need to produce hollow mineral fibers, both by the rotary process and by the continuous fiber process. Hollow fibers would be stronger and lighter than the equivalent solid fibers, and would provide improved resistance to heat flow for thermal insulation products.
One attempt to make non-circular glass fibers was by Warthen, as described in U.S. Pat. No. 3,063,094. Warthen's method employes mechanical perturbation of the glass stream while it is still in a plastic, deformable state. Warthen teaches that to create a non-circular fiber, the glass stream, initially in a conical shape with a circular cross-section, should be distorted at a region where the viscosity of the stream is sufficiently high as to become rapidly chilled or solidified during attenuation of the streams to a continuous fiber whereby a similar distortion in the cross-sectional configuration is retained in the attenuated solidified fiber. Warthen also teaches that a heat sink is to be applied to the glass stream by direct contact. This raises the viscosity of the molten glass to better enable retention and perpetuation of the non-circular cross-sectional character of the mechanically perturbed molten glass stream.
In the art of producing organic fibers, it is a common practice to use quenching methods to solidify molten streams of organic material into non-circular cross-sections which are similar to the shapes of the non-circular orifices. However, these methods are practical under conditions which differ greatly from conditions associated with forming mineral fibers. The production of organic non-circular fibers can be facilitated by pressurization of the bushings, whereas pressurization of bushings containing molten glass presents severe operating problems. The melting points of glass and organic compositions differ by 1500.degree. F. (815.degree. C.) or more. The mineral material of this invention will have a liquidus temperature greater than about 1200.degree. F. (649.degree. C.), whereas organic compositions soften and/or decompose at much lower temperatures.
The differences in physical characteristics can be clearly understood by comparing the ratio of viscosity-to-surface tension for glass with the same ratio for organic fiber forming material. The viscosity-to-surface tension ratio (poises/(dynes/cm)) of polymers lies within the range of from about 25 to about 5000. The ratio for glass is within the range of from about 0.1 to about 25, preferably within the range of from about 0.25 to about 5, and most preferably within the range of from about 0.4 to about 4. The viscosity of molten glass at fiber forming temperatures is typically about 300 poises whereas the viscosity of the molten organic material is typically on the order of about 1000 to about 3000 poises. Also, the surface tension forces of glass (on the order of about 250 to about 300 dynes/cm) are an order of magnitude greater than those of the organic material (about 30 dynes/cm). The lower viscosity and higher surface tension of glass make it about 100 times more difficult to prevent the shaped glass fibers from re-forming into glass fibers having circular cross-sections.
In spite of past attempts to manufacture non-circular and hollow mineral fibers, there has never been a commercially successful method or apparatus for achieving the goal of making hollow fibers or non-circular fibers from non-circular orifices.
STATEMENT OF THE INVENTION
It has now been found that mineral fibers, such as glass fibers, with improved properties can be produced with hollow and/or non-circular cross-sections by discharging primary streams of molten mineral material from orifices in a bushing wall or in a spinner peripheral wall and coalescing the primary streams to each other, or coalescing portions of each primary stream to itself, to form a coalesced stream of different cross-sectional shape from that of the orifices, thereby producing a mineral fiber having a cross-sectional shape similar to the shape of the coalesced stream. The invention can be employed with groups of orifices forming primary streams which are joined to form a coalesced stream having a non-circular shape. Also, hollow fibers can be produced by joining separate primary streams, or by joining the lobes or ends of individual streams into a circular or hollow shape. In order to successfully produce mineral fibers having non-circular cross-sections, the coalesced streams must be quenched sufficiently fast, as by forceable convection cooling, to harden them into non-circular mineral fibers. Also, when used with the rotary process, the invention must be accompanied by the absence of external heating means, or at least a substantial reduction in heat from external heating means.
According to this invention, there is provided a method for making mineral fibers comprising discharging molten mineral material as primary molten streams from orifices positioned in the wall of a container for holding a body of molten mineral material, joining at least one primary stream to an adjacent primary stream to form a coalesced stream of different cross-sectional shape from that of the orifices, and hardening the coalesced stream into a mineral fiber having the approximate cross-sectional shape similar to the shape of the coalesced stream.
In a preferred embodiment of the invention, the external perimeter of the cross-section of the mineral fiber is non-circular.
In another preferred embodiment of the invention, the joining step forms a coalesced stream having a hollow cross-sectional shape, and the hollow coalesced stream is hardened into a hollow mineral fiber.
In a specific embodiment of the invention, the external perimeter of the cross-section of the mineral fiber is non-circular.
According to this invention, there is provided a method for making mineral fibers comprising discharging molten mineral material as multilobal primary molten streams from orifices positioned in the wall of a container for holding a body of molten mineral material, joining a lobe of a primary stream to an adjacent lobe of the primary stream to form a coalesced stream of different cross-sectional shape from that of the orifices, and hardening the coalesced stream into a mineral fiber having a cross-sectional shape similar to the shape of the coalesced stream.
According to this invention, there is also provided a mineral fiber produced according to the method of the invention.